Both hot melt adhesives and moisture curable hot melt polyurethane adhesives are well known generic adhesive classes. A hot melt adhesive can be conveniently applied by extruding the adhesive at an elevated temperature directly onto a work piece to form a structural bond with another work piece as the temperature of the adhesive cools. While hot melt adhesives have many adhesive preparation and work piece production benefits, they have an effective temperature use range that is lower than their application temperature. In other words, the adhesives can lose bond strength as the temperature of the work piece and the bond line increase.
In sharp contrast, moisture curable hot melt polyurethane adhesives have little green strength in their open state, limited peel and shear strength after set and require curing before these properties improve. After application of an adhesive, the joined work pieces may require external mechanical support until the adhesive cures to a strong resilient, crosslinked bond line. A cured polyurethane adhesive bond has high tensile strength, which can be maintained above its application temperature e.g., from about 100° C. to about 130° C. Such adhesives have value where initial green strength is not important since substantial bond formation requires a period of curing time, which can range from hours to a number of days.
Clearly a moisture curable hot melt adhesive that displays the high initial peel and shear strength of a hot melt and that builds in bond strength over time is desirable. Early attempts to formulate such an adhesive have been made but none have been entirely successful. Some early formulas set as a hot melt adhesive, but produced only modest increases in heat resistance when fully cured. Other early adhesives may have good green strength, but short open times, low heat resistance as cured, (e.g., low peel adhesion failure temperature (PAFT)), and limited adhesion to plastics. Accordingly, a substantial need exists in finding better moisture curable hot melt adhesive formulations.